Learning Rigid Body Constraints (Slider & Piston) - Active object keeps passing through passive?
You are trying to get the objects 'Active1' and 'Passive (connected to Active1)' to collide. These two objects share a rigid body constraint through the empty 'Piston Constraint.' This constraint has 'Disable Collisions' checked. This is telling Bullet to ignore 'Active1' and 'Passive (connected to Active1)' collisions.